It is well known that electrical circuit packs require that signals be supplied to them at start-up in a predetermined sequence so as to prevent damage to the circuit pack as well as to obtain proper initialization. Such signals are typically supplied via interconnected connectors and include, but are not limited to, dynamic signals, e.g., time-varying control signals, clock signals and data signals, as well as static signals, e.g., operating potentials and time-fixed control signals. A sequencing mechanism that can control the application of signals to the circuit pack must be incorporated either directly into the interconnection apparatus itself or into a control mechanism external to the circuit pack. Control mechanisms, external to the interconnected connectors, cannot be employed when a connector of a circuit pack is to be plugged into an already active connector, i.e., a connector that is already receiving the signals for transmission to the circuit pack. Such active connectors are typically employed in protection switching arrangements in which a failed circuit pack has been bypassed and must be replaced without shutting down the entire system to which the active connector is connected.
A prior mechanism for sequencing the signals applied to a circuit pack that is not external to either connector and can be used with active connectors, employs conductive fingers of different lengths which form a first connector at an edge of the circuit pack known as an edge connector. The application of signals from the active connector to the circuit pack occurs as the edge connector of the circuit pack is inserted into the active connector. Each signal supplied from the active connector to the circuit pack is applied to the edge connector in accordance with a desired predetermined order that is dependent upon the lengths of the various conductive fingers of the edge connector that transport input signals. Similarly, signals that are to be supplied from the circuit pack to the active connector are initially supplied from the edge connector in accordance with a desired predetermined order that is dependent upon the lengths of the various conductive fingers of the edge connector transporting output signals. Other prior sequencing methods that employ conductive pins, typically capitalize upon predetermined variations in the lengths of the pins whereby the interconnection of signals from between the first and second connectors is controlled.
Some prior protection switching arrangements also require that certain signals pass continuously through the active socket from the input point of the signal to the output point of the signal with permissible interruptions of only a short and fixed duration if the signal is not being processed. To maintain a continuous signal flow, an input point and an output point for such a signal are located opposite each other in the connector and spring action shorting contacts are employed as the contacts in the active connector. When the active connector is not interconnected to another connector, the shorting contacts are forced closed, thereby touching each other and creating an electrical connection between them, i.e., a short circuit connection. A signal supplied to a first one of the contacts of the shorting contacts, designated the input contact, flows directly, when the shorting contacts are closed, to the opposing one of the shorting contacts, designated the output contact. Upon insertion of another connector, e.g., one connected to a circuit pack, into the active connector the shorting contacts are forced apart. Each contact of the shorting contacts can contact independent conductive terminals of the connector to which the active connector is mated. Thereafter, any desired processing of the signal supplied to the circuit pack from the input contact of the shorting contacts can be performed by the circuit pack. The resulting processed signal may be supplied as an output to the output contact of the shorting contacts via the opposing conductive terminal, and/or it may be supplied to any other individual or combination of conductive contacts in the active connector by the opposing conductive terminals, dependent on the implementor's discretion. Alternatively, no processing of the input signal can be performed, e.g., a direct connection can be established between the conductive terminal of the connector of the circuit pack receiving the signal as an input from the input contact of the shorting contacts and the corresponding conductive terminal of the connector supplying a signal as an output to the output contact of the shorting contacts via a relay in the event of circuit pack failure. Such a configuration is typically employed in protection switching.
Interruptions in the supply of signal from the input contact to the output contact of the shorting contacts typically take place upon mating or disconnection of a first connector into an active connector because the time of "make", i.e. the time at which a good electrical connection is made between the conductive terminal of interest on the first connector and the active connector, is later than the time of "break", i.e. the time at which the electrical connection is broken between the shorting contacts. Such a time differential is largely caused by the interposition of an insulating material inherent in the first connector itself which causes the shorting contacts to open prior to their reaching the first point on the first connector to which the conductive material comprising the terminal element reaches. This early opening of the shorting contacts is an undesired side effect of having the terminal elements which are to make contact with the shorting contacts be of a shorter length than some of the other terminal elements while retaining the presence of the insulating portion of the connector material so as to also achieve other desirable goals. Such goals include, without limitation: ease of manufacture, reduced manufacturing costs, enhanced mechanical reliability and ability to use with some insertion force reduction techniques. Additionally, such a connector, upon mating with or disconnecting from the active connector, can be placed so that the shorting contacts remain open and no contact is established between the shorting contacts and the conductive terminal element of the first connector.
Another problem arising from the use of shorting contacts and predetermined variable length conductors with edge connectors so as to achieve sequencing is a limitation of some of the commercially available connectors and circuit packs to only three guaranteed levels of sequencing, i.e., a maximum of three conductive finger lengths can be distinguished. Additionally, it is economically desirable to keep the number of available sequencing levels to a minimum so as to employ less precise tolerances and minimize the overall size of the interconnection apparatus. As mentioned, many typical applications require relays configured so as to allow a signal of interest to bypass the circuit pack in the event of failure of the circuit pack. A typical sequencing assignment is for the first level of signals, i.e., those signals carried by the longest conductive fingers, to be assigned to ground potential. The second level, i.e., those signals carried by the next longest conductive fingers, are assigned to power, typically +5 volts, and the signal controlling the state of the relay (relay arbitration). The third level, i.e., those signals carried by the shortest conductive fingers, are assigned to all other signals, including the signal of interest which passes through the shorting contacts and the contacts of the relay.
The relay is opened and closed under the control of a relay driver circuit which is responsive to the aforementioned signal controlling the relay arbitration. The input lead for receiving the signal controlling the relay arbitration is diode clamp protected against damage by excessive voltage transients. However, if the input lead is diode clamped to the power lead, a race condition occurs (dependent on the angle of board insertion, trace layout and the electrical characteristics of the power and relay arbitration control signal) whereby if the relay control signal wins the clamping diode begins to conduct. This causes the signal controlling the relay arbitration to appear at the power bus of the circuit pack. The circuit pack then attempts to power its circuitry from the power bus by drawing, in turn, large amounts of power through the relay control signal lead. The drawing of such amounts of power can damage the supplier and/or receiver of the signal controlling the relay arbitration and the relay driver circuit.